In a conventional homodyne interferometer system, if a phase difference between a reference light (R) and a signal light (S) is represented as θ, an interferometer output (I) is represented by the following formula:I=R+S+2√{square root over ( )}(R·S)cos θ(I, R and S are the laser power values.)θ=arccosine((I−R−S)/2√{square root over ( )}(R·S))
In the conventional homodyne interferometer system, a sensor output signal is affected by fluctuation of a laser signal generator output level in accordance with temperature drift, which results in variation in an optical fiber transmission path and residual errors in the output value of the sensor output signal because the output value is calculated according to the condition that the reference light and the signal light intensities are constant.
Although the conventional homodyne interferometer system is able to detect the phase difference of a half of light wavelength between the reference light and the signal light, it is almost impossible to keep the phase difference between the reference light and the signal light within this narrow range because the components of the conventional homodyne interferometer system do not have excellent dimension accuracy in order to keep the phase difference into the narrow range. Moreover, it is impossible to eliminate, in the conventional homodyne interferometer system, the optical path fluctuation by the influence of temperature variation in the surrounding environmental. Further, it is noted that it is noted possible to extend the this limited measurement range in the conventional interferometer system.
Based on the above-noted deficiencies in the conventional homodyne interferometer system, an optical fiber sensor that shows good insusceptibility to a strong electromagnetic field and an extremely high or low temperature environment is desired, in order to provide an optical fiber sensor for low transmission loss over long distances.